The aim of our work was to study in a population of high risk patients with hemorrhagic and or thrombotic disease, the preventive or therapeutic effect of a low molecular weight heparin fraction, CY 216 (Choay, France), particularly in surgery. CY 216 was given to 9 patients for the treatment of a thrombosis (pulmonary embolism, acute ischemia, deep venous thrombosis) and to 40 patients in prevention of thrombosis. In this second group, 28 had a high thromboembolic risk such as valvular prosthesis, cardiac arrythmia, coronary artery bypass, etc. For all the patients, CY 216 was injected sub-cutaneously twice or three times a day at the mean dose of 1.5 mg/kg/d, equivalent to 300 U anti-Xa Choay/24 h, and always injected 24 hours before surgery. The biological tests used were: blood cells count, platelet count, prothrombin time, activated partial thromboplastin time, heparinemia levels by two technics: anti-factor-Xa activity and anti-factor IIa activity. None thrombotic complication was observed in the 40 patients prophylactically treated and a constant improvement of thrombosis was noted for the 9 patients with thrombo-embolic disease. In 3 patients, bleeding complications were observed: for 2 patients, all the coagulation tests were normal and anti-Xa activities were less than 0.55 U/ml; in one patient, the bleeding time was prolonged (15 minutes Ivy Incision) and returned to normal when the CY 216 was stopped. Concerning the biology, there was no modification except for anti-Xa activity which mean was 0.30 U/ml (01-07). However, this test is unable to predict either thrombotic or hemorrhagic events.